1. Field of the Invention
The invention is related to a method for improved growth of semipolar (Al,In,Ga,B)N.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques comprising molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
GaN and its alloys are most stable in the hexagonal würtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the würtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the würtzite structure exhibits piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn gives rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on nonpolar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent nonpolar layers are crystallographically equivalent to one another so the crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent nonpolar planes in GaN are the {11-20} family, known collectively as a-planes, and the {1-100} family, known collectively as m-planes. Unfortunately, despite advances made by researchers at the University of California, the assignee of the present invention, growth of nonpolar nitrides remains challenging and has not yet been widely adopted in the III-nitride industry.
Another approach to reducing or possibly eliminating the polarization effects in GaN optoelectronic devices is to grow the devices on semipolar planes of the crystal. The term semipolar planes can be used to refer to a wide variety of planes that possess two nonzero h, i, or k Miller indices, and a nonzero 1 Miller index. Some commonly observed examples of semipolar planes in c-plane GaN heteroepitaxy include the {11-22}, {10-11}, and {10-13} planes, which are found in the facets of pits. These planes also happen to be the same planes that the authors have grown in the form of planar films. Other examples of semipolar planes in the würtzite crystal structure include, but are not limited to, {10-12}, {20-21}, and {10-14}. The nitride crystal's polarization vector lies neither within such planes or normal to such planes, but rather lies at some angle inclined relative to the plane's surface normal. For example, the {10-11} and {10-13} planes are at 62.98° and 32.06° to the c-plane, respectively.
In addition to spontaneous polarization, the second form of polarization present in nitrides is piezoelectric polarization. This occurs when the material experiences a compressive or tensile strain, as can occur when (Al,In,Ga,B)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a strained AlGaN layer on a GaN template will have in-plane tensile strain, and a strained InGaN layer on a GaN template will have in-plane compressive strain, both due to lattice matching to the GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer latticed matched to GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN.
The advantage of using semipolar planes over c-plane nitrides is that the total polarization will be reduced. There may even be zero polarization for specific alloy compositions on specific planes. The important point is that the polarization will be reduced compared to that of c-plane nitride structures.
Bulk crystals of GaN are not readily available, so it is not possible to simply cut a crystal to present a surface for subsequent device regrowth. Commonly, GaN films are initially grown heteroepitaxially, i.e. on foreign substrates that provide a reasonable lattice match to GaN. Common substrate materials are sapphire (Al2O3) and spinel (MgAl2O4).
Large crystals of such substrate materials may be made by those practiced in the art. The crystals are then cut into substrate wafers, where the wafer surface has a specific crystallographic orientation, conventionally specified by Miller indices (hkl). Typically, low index crystal orientations are chosen which match the crystal symmetry of the material to be deposited on them. For example, (0001) sapphire substrates, which possess a hexagonal in-plane symmetry, are used for the growth of conventional polar nitride layers, which also possess a hexagonal in-plane symmetry. The existence of a crystallographic relationship between the substrate and deposited layer or layers is termed epitaxy.
Further, the heteroepitaxial growth of a nitride layer on a foreign substrate must first begin from small nuclei consisting of a few atoms. The energy of nuclei formed on a flat atomic surface is higher than that of nuclei formed at atomic steps or kinks, because the steps or kinks minimize the surface energy of the nuclei. Intentionally miscutting the substrate crystal away from a low index plane (hkl) produces step edges and kinks. Such a miscut surface orientation is termed a vicinal surface.
FIG. 1 shows a schematic representation of a vicinal surface with atomic steps or kinks. The miscut angle, β, is defined as the angle between the surface normal, n, and the primary crystal orientation [uvw], denoted by g. Substrates may be cut from a bulk crystal by those practiced in the art with a specific magnitude of miscut angle. Further, the direction of the miscut vector g may be specified relative to a specific in-plane crystallographic direction [uvw], as denoted by the angle α in FIG. 1.
Semipolar GaN planes have been demonstrated on the sidewalls of patterned c-plane oriented stripes. Nishizuka et al. [1] have grown {11-22} InGaN quantum wells by this technique. They have also demonstrated that the internal quantum efficiency of the semipolar plane {11-22} is higher than that of the c-plane, which results from the reduced polarization.
However, Nishizuka et al.'s method of producing semipolar planes is drastically different from that of the present invention, because it relies on an artifact of the Epitaxial Lateral Overgrowth (ELO) technique. ELO is a cumbersome processing and growth method used to reduce defects in GaN and other semiconductors. It involves patterning stripes of a mask material, often silicon dioxide (SiO2) for GaN. The GaN is then grown from open windows between the mask and then grown over the mask. To form a continuous film, the GaN is then coalesced by lateral growth. The facets of these stripes can be controlled by the growth parameters. If the growth is stopped before the stripes coalesce, then a small area of semipolar plane can be exposed, typically 10 μm wide at most, but this available surface area is too small to process into a semipolar LED. Furthermore, the semipolar plane will be not parallel to the substrate surface, and forming device structures on inclined facets is significantly more difficult than forming those structures on normal planes. Also, not all nitride compositions are compatible with ELO processes and therefore only ELO of GaN is widely practiced.
The present invention discloses a method allowing for the growth of planar films of semipolar nitrides, in which a large area of (Al,In,Ga,B)N is parallel to the substrate surface, through the use of intentionally miscut substrates. For example, samples are often grown on 2 inch diameter substrates compared to the few micrometer wide areas previously demonstrated for the growth of semipolar nitrides.
A paper has been published where a thick c-plane GaN crystal was grown by HVPE, subsequently cut and polished on the {11-22} plane [2]. A light emitting diode was then grown on this plane. However, this method for fabricating a semipolar device is drastically different from the preferred embodiment of this invention. The above mentioned method uses a bulk GaN substrate for which a GaN semipolar surface has been exposed and is used for subsequent deposition of the device structure, otherwise known as homoepitaxy. One of the key features of the preferred embodiment of this invention is the use of a heteroepitaxial process, by which a substrate of a different material is used to produce a semipolar nitride film. This invention also distinguishes itself from the above mentioned process by allowing the use of a large, typically 2 inch, wafer in which the entire area is a semipolar film. This is in sharp contrast to the above mentioned method in which the semipolar film is only typically 4 mm by 10 mm is size, due to the unavailability of large area GaN crystals.
Growth of semipolar orientations of (Al, In, Ga)N thin films does not eliminate the total polarization of the semiconductor crystal; however, the growth of semipolar orientations of (Al, In, Ga)N thin films mitigates discontinuities in the total polarization along the growth direction of semiconductor device structures fabricated from these layers. Intentionally miscut substrates have been employed during the epitaxial growth of semiconductor thin films to improve surface morphology and/or crystal quality. In the case of GaN, see, for example, Hiramatsu, et al. [3], or Grudowski, et al. [4] However, the use of an intentional miscut has not been employed to control the relative orientation of the polarization field in (Al, In, Ga)N semiconductor thin films for the mitigation of polarization-related effects in (Al, In, Ga)N heterostructures.
Miscut substrates have in general been used for the growth of semiconductor thin films. This holds true for both homoepitaxy and heteroepitaxy of semiconductor films.